Thermal wave and plasma wave monitoring systems typically are based on the detection of changes in intensity of a probe beam reflected off the surface of a semiconductor or other appropriate sample. Changes in the reflectivity of the surface are caused by thermal and plasma waves arising as a result of the absorption of an intensity-modulated pump beam directed at or near the same area on the surface as the probe beam. This technique is known in the prior art as the modulated optical reflectance (MOR) or thermal wave (TW) method. Exemplary thermal wave and plasma wave monitoring systems are described in U.S. Pat. Nos. 4,636,088, 4,854,710, and 5,978,074, each of which is hereby incorporated herein by reference. An exemplary optical arrangement of the prior art for capturing TW information is shown in FIG. 1, and described more fully in U.S. patent application Ser. No. 10/796,603, Publication No. 2004/0251927, which is hereby incorporated herein by reference. The system 100 includes a pump laser 102 and a probe laser 104, where the pump laser intensity is varied to create an intensity-modulated pump beam that is projected against the surface of a sample and absorbed, causing localized excitation of the sample. As the pump laser is modulated, the localized excitation (and subsequent relaxation) creates a train of thermal and plasma waves within the sample. These waves reflect and scatter off various features and interact with various regions within the sample in a way that alters the flow of heat and/or plasma from the pump beam spot. The presence of the thermal and plasma waves has a direct effect on the surface reflectivity of the sample. Features and regions below the sample surface that alter the passage of the thermal and plasma waves will therefore alter the optical reflective patterns at the surface of the sample. By monitoring the changes in reflectivity of the sample at the surface, information about characteristics below the surface can be obtained.
To monitor the surface changes, a probe laser is used to direct a probe beam at a portion of the sample that is excited by the pump laser. The sample reflects the probe beam and a photodetector 106 records the intensity of the reflected probe beam. The output signal from the photodetector is filtered to isolate the changes that are synchronous with the pump beam modulation. The detector generates separate “in-phase” (I) and “quadrature” (Q) outputs that can be supplied to a processor 108 and used to calculate amplitude and phase of the modulated signal. The amplitude and phase values are used to deduce physical characteristics of the sample. In most cases, this is done by measuring amplitude values (amplitude is used more commonly than phase) for one or more specially prepared calibration samples, each of which has known physical characteristics. The empirically derived values are used to associate known physical characteristics with corresponding amplitude values. Amplitude values obtained for test samples can then be analyzed by comparison to the amplitude values obtained for the calibration samples.
As part of the manufacturing process, ions (or dopants) are added to the near-surface region of semiconductors using a process known as implantation. The implanted region (with its relatively high dopant concentration) overlays a non-implanted region where dopant concentrations are relatively low. Incompleteness of anneal, a parameter that is crucial to USJ characterization, appears when non-uniformities in structural damage caused by ion implantation along with malfunctioning of the rapid thermal anneal process and other types of annealing processes result in residual structural damage areas on the surface of a semiconductor wafer after anneal. This incomplete anneal should also be monitored to increase manufacturing yield and to ensure high performance characteristics of a semiconductor device.
Damage relaxation after ion implantation, as well as incomplete anneal due to residual damage and/or surface states, results in a gradual change in the measured TW signal on a semiconductor element, such as a silicon wafer, as a function of time. If measurements are made on a wafer immediately after implantation or anneal, and at periodic intervals thereafter, the TW signal will slowly change until a steady-state asymptotic value is reached. The amount of change in the TW signal, as well as the characteristic time for the signal to stabilize, will depend on factors such as the properties of the semiconductor wafer, as well as the implantation and anneal conditions.
One prior art technique eliminates the dependence of the TW signal on time. This technique is described, for example, in U.S. Provisional Application Ser. No. 60/495,053, filed Aug. 14, 2003, entitled “METHOD FOR COMPENSATING FOR INCOMPLETE ANNEAL IN ION-IMPLANTED SEMICONDUCTORS,” which is hereby incorporated herein by reference. This technique allows for a characterization of anneal process, including measurements of completeness and uniformity, as well a compensation for incomplete anneal and damage relaxation. This technique utilizes a pump laser to accelerate the annealing process. When the pump beam illuminates a single spot, such as a 1 μm spot, on a semiconductor wafer subject to damage relaxation or having residual damage, the thermally-induced crystal restructuring effect can be accelerated so that a steady-state “annealed” signal can be obtained in a matter of seconds, rather than the hours or even days necessary to obtain a steady state signal at room temperature (i.e., an “environmental” anneal). Such an annealed TW signal is independent of the time elapsed since the implant and/or anneal processes were performed.
FIG. 2 shows an exemplary TW signal as monitored and recorded over time. The resultant curve 200 can be fit to an exponential decay, and an anneal decay factor (ADF) can be calculated. The ADF parameter characterizes the degree of damage relaxation or completeness of the anneal, and can be used to compensate for these phenomena. The anneal decay factor in this example is calculated from the following relation:ADF=TW10/TW0 where TW0 corresponds to the value of the TW signal at the beginning of the compensation process, and TW10 corresponds to the value of the TW signal recorded and after 10 seconds of the compensation process. While 10 seconds can be appropriate in this example, other periods of time can be used that can be appropriate for different conditions, such as different implant conditions. Further, the decay can be measured for a period of about 10 seconds then extrapolated to a period of about 30 seconds, for example. A separate ADF value can be calculated for each individual set of implantation and anneal conditions to characterize the completeness of the anneal process.
When characterizing the anneal process, it typically cannot be assumed that the annealing process is uniform across an entire surface. Non-uniformities can be more common when using a rapid thermal anneal (RTA) process. In this case, the ADF technique can monitor a degree of anneal non-uniformity across the wafer by measuring point-by-point TW0 and TW10 contour maps. FIG. 3 shows (a) TW0 and (b) TW10 maps 300, 310 obtained from the same region on a semiconductor wafer after anneal, where regions of substantial residual damage (e.g., incomplete anneal, RTA failure) can be seen. From these maps, an ADF contour map 400 can be calculated, as shown in FIG. 4. The ADF contour map can be used to characterize the quality of anneal process. Such a map implies that the closer the ADF is to unity, the better the annealing quality.
Such a technique can be used to monitor and/or control uniformity and completeness of anneal after each technological step in a fabrication process. In this case the ADF measurement results obtained after each process step, whether at individual sites or entire wafer maps, can be analyzed and compared, thus facilitating process system troubleshooting in applications such as semiconductor manufacturing.
One of the most important characteristics of the TW system performance in pre- or post-anneal applications is the repeatability of the measurements. Wafer non-uniformities, as well as TW system drift and other variables, can result in changes in the TW signal over time. Single-point measurements at several locations across the wafer surface cannot produce a desirable repeatability over time, even when supported by an ADF characterization.